1. Field of the Invention
The present invention relates to a monitor system, and more particularly, to a dynamic focusing circuit of a monitor system for effectively compensating a defocusing of a screen which often occurs in the monitor system which adopts a cathode-ray tube (hereinafter, refer red to as "CRT").
2. Description of the Prior Art
In general, the CRT of the monitor system has an electron gun for emitting an electron beam, an anode for accelerating the electron beam emitted from the electron gun to be moved to the front of the CRT, a deflecting yoke (hereinafter, referred to as "DY") for deflecting the accelerated electron beam to determine a moving direction of the electron beam, a negative electrode for emitting hot electrons in response to a red-green-blue video signal (hereinafter, referred to as "RGB video signal") input to the CRT and grids for supplying the hot electrons emitted from the negative electrode to a fluorescent screen as the electron beam. At this time, luminescent materials corresponding to the each of the red, green and blue colors are regularly arranged on the fluorescent screen. The video displayed on the CRT is formed by light generated by means of colliding the electron beam passed the grids with the luminescent materials arranged on the fluorescent screen.
However, the more a deflecting angle, according to the DY, enlarges, the more a diameter of the electron beam of the CRT enlarges in general. That is, the more the electron beam becomes distant from the center of the fluorescent screen, the more the diameter of the electron beam which collides with the luminescent materials enlarges. Accordingly, the focusing depth of the video displayed on the screen decreases. To prevent the focusing depth from decreasing, a method which adopts a dynamic focusing circuit for stably supplying a voltage applied to the grid corresponding to a focusing electrode among the grids has been disclosed.
However, such dynamic focusing circuit can not correctly compensate the defocusing due to delay and distortion of the signal caused by a stray capacitance of a transfomer. Also, in a case where a user changes vertical or horizontal size of the screen of the monitor system, it is not able to compensate the defocusing because waveforms of vertical and horizontal parabolic signals applied to the dynamic focusing circuit are changed.